Hygroelastic Transition: Unusual Non-Equilibrium Behavior Found in Hygroscopic Biological Materials with Rich Characteristics*

Recent experiments on the hygroscopic spores of a common soil bacterium have unveiled unusual equilibrium and nonequilibrium properties that do not have counterparts in other types of solid matter (1). Among these is a marked nonequilibrium transition in mechanical properties at short timescales called the hygroelastic transition, which was predicted to originate from jamming of water molecules confined between biomolecules. Several predictions about this transition remain to be tested. Specifically, the hygroelastic theory predicts that (i) the observed transition in elastic modulus should be accompanied by a transition from a strong nonlinear elastic behavior to a linear elastic behavior, (ii) the transition timescale should depend strongly on equilibrium mechanical strain, and (iii) the transition should be observable in other types of hygroscopic biological materials. Here we present evidence for these three predictions with atomic force microscopy measurements on bacterial spores and regenerated cellulose films. Our experiments are based on frequency-dependent contact stiffness measurements. Findings suggest that the hygroelastic transition is an unusual nonequilibrium phenomenon with a rich set of features.

1. Harrellson, S.G. et al. Hydration solids. Nature 619, 500–505 (2023).

This work was supported by National Institute of General Medical Sciences of the National Institutes of Health, award no R35GM145382 and by the Office of Naval Research, award no. N00014-21-1-4004.